1. Field of the Invention
This invention relates to a surface mountable electrical device, more particularly to a surface mountable electrical device that can serve as a circuit protection device, such as a fuse.
2. Description of the Related Art
FIG. 1 illustrates a conventional surface mountable electrical device (PTC thermistor) which includes a plurality of stacked PTC sheets 4 that have a positive thermal coefficient characteristic, a plurality of metal foil sheets 21, 23 alternately laminated with the PTC sheets 4, a first electrode layer 22 formed on an upper surface of the device, a second electrode layer 24 formed on a bottom surface of the device, a conductive first side layer 51 interconnecting the first electrode layer 22 and one of the metal foil sheets 23, and a conductive second side layer 52 interconnecting the second electrode layer 24 and another one of the metal foil sheets 21. Commercially available metal foil sheets, which are made from copper, normally have been surface treated at one side face so as to be attachable to a contacting surface of a PTC sheet. However, the other side face of each of the metal foil sheets is not surface treated and is relatively smooth. As a result, the non-surface-treated side face of each of the aforesaid metal foil sheets 21, 23 is in poor contact with the respective PTC sheet 4, and thus tends to peel from the respective PTC sheet 4.
In order to overcome the aforementioned drawback, U.S. Pat. No. 6,157,289 disclosed a surface mountable electrical device (see FIG. 2) that includes a metal foil sheet 11, which is sandwiched between two stacked PTC sheets 14 and that is surface treated by being plated with nickel layers 12 on opposite side faces thereof so as to roughen the side faces and so as to permit secure attachment of the same to contacting surfaces of the PTC sheets 14. However, the extra processing step of forming the nickel layers 12 significantly increases the cost of manufacture. Moreover, the metal foil sheet 11 is required to be formed with grooves along cutting lines by etching or die press prior to being laminated with the PTC sheets 14 for facilitating formation of pieces of the surface mountable electrical device in a subsequent cutting step.
Therefore, the object of the present invention is to provide a surface mountable electrical device that is capable of overcoming the aforementioned drawbacks.
According to the present invention, a surface mountable electrical device comprises: a laminate body having top and bottom faces and opposite first and second side faces transverse to and interconnecting the top and bottom faces, the laminate body including a plurality of stacked PTC sheets which have a positive thermal coefficient characteristic, two overlapping metal foil sheets sandwiched between each two adjacent ones of the PTC sheets and having contact ends that extend to and that are exposed from the first side face, a conductive first electrode layer formed on the top face, a conductive second electrode layer formed on the top face and spaced apart from the first electrode layer, and a conductive third electrode layer formed on the bottom face, the two adjacent ones of the PTC sheets having contact faces with inlaid portions respectively inlaid with the metal foil sheets, and non-inlaid portions bonded to each other; and at least a conductive transverse layer formed on the first side face and interconnecting the first and third electrode layers and the contact ends of the metal foil sheets.